Process for forming a planar diode using one mask

ABSTRACT

A planar diode and method of making the same employing only one mask. The diode is formed by coating a substrate with an oxide, removing a central portion of the oxide to define a window through which dopants are diffused. The substrate is given a Ni/Au plating to provide ohmic contact surfaces, and the oxide on the periphery of the window is coated with a polyimide passivating agent overlying the P/N junction.

BACKGROUND OF THE INVENTION

The present invention relates to a wafer-level process for fabricatingsemiconductor planar devices such as diodes and, more particularly,concerns a simplified wafer level process for fabricating planar diodesfrom semiconductor.

Diodes are mass produced in great numbers every year. As a high volumecomponent found in virtually every electrical device of any complexity,the market for diodes is very large, competitive, and sensitive topricing pressure. Manufacturers must produce these devices both withsufficient reliability and low cost to satisfy the competitive demandsof the marketplace. Currently, one known process for manufacturingdiodes from semiconductor chips entails the use of severalphotolithographic steps employing a mask, each of which increases thecost of manufacture.

A specific example of one such known process is a standard glasspassivated pallet process employing three masking steps, and isillustrated in the process flow diagram of FIG. 1. Beginning in step 1with a wafer of silicon 10, the wafer is doped in step 2 so as toprovide P+, N, and N+ regions (respectively indicated as 12, 14, and 16in FIG. 1). Prior to the first photolithographic step 3, a protectiveoxide layer is provided in the form of an oxide coating 18. Afterbaking, developing, and hard baking, a first mask is used to generatethe structure shown in the figure after step 3, in which windows 19 areopened up in the oxide layer 18. In step 4 these windows are etched toform grids in the silicon wafer, defining the intended boundaries ofeach diode.

In step 5, a layer 20 of polynitride is deposited to prepare the surfacefor step 6, the second photolithographic step in which a mask isemployed. Here, a layer of glass powder 22 is deposited along the gridas indicated, baked under high pressure and fired (step 7).

In step 8, a low temperature oxide chemical vapor deposition process isused to overlay an oxide (silicon dioxide) on the surface to protect theglass for step 9, the third photolithographic step in which a mask isemployed (here to deposit a polymer coating).

Contact etching and photoresist etching is performed in step 10,exposing the P+ and N+ surfaces, which are then coated with nickelplating in step 11 to provide ohmic contacts.

This known process requires three photolithographic steps in which aprecision mask is employed. The repeated use of masks and the care withwhich they must be used is a substantial component in the cost of thefinished product produced by this process.

Demand persists for diodes that can be manufactured using simpler andcheaper processes

SUMMARY OF THE INVENTION

The present invention includes embodiments that provide a planar diodeand methods for its manufacture. The device comprises: (a) a substrateof a first conductivity type (preferably an N-type conductivity siliconsubstrate) and a doped, centrally located region of a secondconductivity type (preferably a P+-type region) defining a P/N junctiontherebetween; (b) a nickel plating on the underside of the substrate andalong the doped region of the substrate which corresponds to the secondconductivity type; (c) an oxide coating on the peripheral portion of thetop side of the substrate adjacent the doped central portion; and (d) acoating of a passivating material such as polyimide on top of the oxideon the top side of the substrate, the passivating material extendingpartially over the P/N junction. Boron may be employed as a dopant inthis invention.

According to another embodiment of the invention, a method of forming aplanar diode is provided. The method comprises: (a) providing asubstrate of a first conductivity type; (b) depositing a layer of oxideover the substrate; (c) using a mask to expose the central portion ofthe oxide layer for etching; (d) removing the central portion of theoxide via etching; (e) forming a P/N junction in the substrate viawindow diffusion; (f) plating nickel onto the window and on the oppositeside of the substrate; and (g) removing the remaining oxide and coatinga portion of the plating on a side of the substrate with a passivatingagent, such as a polyimide.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will become more readily apparent from thefollowing detailed description taken in conjunction with theaccompanying drawings in which:

FIG. 1 is a schematic view of a known process for manufacturing planardiodes.

FIG. 2 is a schematic view of a first embodiment of a process for themanufacture of diodes according to the principles of the invention.

FIG. 3 is a schematic view of a second embodiment of a process for themanufacture of diodes according to the principles of the invention.

DETAILED DESCRIPTION OF THE INVENTION

The present invention is directed both to a process for manufacturingplanar diodes and the diodes so manufactured. One advantage associatedwith the present invention is that it requires the use of only one maskin contrast to other approaches which require more, thereby resulting ina more economical and reliable process for manufacturing planar diodes.Not only does the use of a single mask simplify the process, but itrequires less equipment as well, as only a single conventionalphotolithographic aligner is required by this process. According to oneaspect of the invention, a coating is provided of passivating materialsuch as polyimide in order to enhance the resistance of the device tomechanical and environment stresses and moisture. This results in a moreeconomical and reliable process for manufacturing planar diodes.

FIG. 2 illustrates a process for such manufacture in accordance with anembodiment of the present invention. The process begins at step 101 witha wafer 100, typically made of silicon (although the process can beemployed with other semiconductive materials).

In step 102, the upper surface is oxidized in a known manner to providean oxide layer 110 of silicon dioxide. (The lower surface may optionallybe oxidized as well.)

Next, in step 103, a photoresist 120 is developed for contact etching.This is the only step in the process in which a mask is employed.

In step 104, contact etching exposes a window 112 in the oxide layer.Through this layer, a P/N junction is formed in step 105 via windowdiffusion, thus creating P+, N, and N+ type regions 114, 116, and 118respectively, as is known in the art. Alternatively, differentimpurities can be employed to create a N/P junction comprising N+, P,and P+ type regions.

Ohmic contacts 132 and 134 are provided in step 106 via nickel plating,as is known in the art. The oxide 110 acts as a mask for the depositionof the metal contact 132 and is self-aligned to the P/N junction withoutneed of an additional mask for metallization along the window 112.

By the end of step 106, a functional diode has been produced. However,in order to passivate the surface and thus provide a more reliable anddurable device, a polyimide coating 140 is added in step 107 via thescreen printing method (which is less expensive than the use of a mask).The polyimide coating serves to protect the device, and in particular,the P/N junction, against contamination and moisture. Optionally, and tofurther protect the nickel surface against corrosion, a gold plating maybe applied to the exposed nickel surfaces.

By reducing the number of masks that are employed to one, the resultingplanar diodes are cheaper to manufacture.

According to one example, the dimensions of the diode may be as follows:

Layer Thickness (approx., and in microns) Oxide 2.0 photoresist 5.0 P+50.0 N+ 100.0 Polyimide 10.0 Nickel/Gold 2.0

FIG. 3 illustrates an alternative embodiment of the invention, in whichsteps 201-206 are identical to steps 101-106, both in process and inresulting intermediate structures. This process begins at 201 with awafer. In step 202, the upper surface is oxidized in a known manner toprovide an oxidized layer. In step 203, a photoresist 120 is developedfor contact etching. This is the only step in the process in which amask is employed. In step 204, contact etching exposes a window in theoxide layer. Through this layer, a P/N junction is formed in step 205via window diffusion, thus creating P+, N, and N+ type regions. Ohmiccontacts are provided in step 206 via nickel plating.

The process diverges from that shown in FIG. 2 in step 207, in which theremaining oxide layer 110 is stripped off. Removing the layer of oxideat this point has the advantage of enabling the removal of contaminantsthat my have infiltrated between the oxide and the silicon, therebyproviding a cleaner P/N junction. Then, in step 208, a passivating layerof polyimide 240 is applied directly to the exposed silicon, overlappingsome with the upper contact 132.

Although various embodiments are specifically illustrated and describedherein, it will be appreciated that modifications and variations of thepresent invention are covered by the above teachings and are within thepurview of the appended claims without departing from the spirit andintended scope of the invention. For example, this approach could beapplied to the manufacture of different kinds of semiconductor devicessuch as transient voltage suppressors, thyristors, and transistors.

What is claimed is:
 1. A method of forming a planar diode comprising:providing a substrate of a first conductivity type; depositing a layerof oxide over the substrate; using a mask to expose the central portionof the oxide layer for etching; removing the central portion of theoxide via etching; diffusing dopants into the substrate via windowdiffusion to form a single region having a uniform second conductivitytype; plating a metal onto the window and on the opposite side of thesubstrate, wherein the oxide acts as a mask for the plating of themetal, and the metal is plated only onto the window and is not platedonto the oxide; and coating the remaining oxide and a portion of theplating on a side of the substrate with a passivating agent, wherein theplanar diode is formed using only one mask.
 2. The method of claim 1,wherein the passivating agent is polyimide.
 3. The method of claim 1,wherein said first conductivity type has N-type conductivity and saidsecond conductivity type has P-type conductivity
 4. The method of claim1, wherein the metal comprises nickel.